Add support for camel casing

[Ekrekr]

Introduce support for additional options that come after a second colon, starting with camelCasing of fields.

• Currently doc_opts takes the form of <type>,<output>:<excludes>
• This PR introduces the new structure of <type>,<output>:<excludes>:<additional options>.
• See the update to the --doc_opt comment for the full new structure details.
• This seems like a clean way to support the new camel_case_fields option, which is both backwards compatible without it being populated, and is forwards compatible with additional options being added.

Additional notes:

• I've implemented additional options parsing in a very simplistic way, splitting by , can be added later if needed.
• Passing plugin options seems like the most extensible way to support further additional options.
• I had to bump the protobuf dependency version for the master branch to build.

Fixes #539 - looking closer, I don't think docs_out is possible, so this implements the desired functionality directly.

[Ekrekr]

@pseudomuto are you the right person to review? Thanks!